The present invention is directed to an illumination system for an LCD projection display system. The invention is specifically directed to liquid crystal projection display systems in which the liquid crystal light modulating cell is of the twisted nematic or field effect birefringement configurations, which require that the light incident at the cell be linearly polarized.
In such systems linearly polarized light is derived from a non-polarized light source and is typically filtered to yield a linearly polarized beam. This polarized beam is directed to the light modulation device. Due to this filtering, at best, only half the light output of the light source is utilized. However, projection display systems are progressing toward systems of greater and greater brightness so as to be usable in a normally lit room. Clearly, systems that permit half of the output of the light source to go unused are inefficient. The present invention is directed towards providing an LCD projection display system which utilizes all of the light from the source by separating the unpolarized light into two orthogonally polarized beams, converting the direction of polarization of one of the orthogonally polarized beams and combining it with the other so as to utilize both of the beams.
An example of an LCD display system in which only one of the orthogonal beams is utilized is shown in the design proposed by Seiko Epson Corp. which is described in the May 12, 1986 issue of "Electronics" at page 47. This system uses an absorption polarizer which absorbs the beam of the unwanted polarization and which transmits typically only 40% of the light provided by the source.
One approach for utilizing all of the light of the light source is described in U.S. Pat. No. 4,127,322 which is directed to an LCD projection television system. The system utilizes six electron beam addressed LCDS. Three LCDS are used for each beam of polarized light from a polarizing beamsplitter. The usefulness of such a system is accordingly limited by its expense and complexity.
Another approach for utilizing all of the light of the illumination system in an LCD projection television system is shown in Japanese Patent Application No. 61-122626 in which the light of the desired polarization direction is passed to the light modulating LCD. The light component of the other polarization direction is reflected by an additional mirror back towards the light source and is reflected again by the source's reflector. The returned light passes through a quarterwave plate twice so that its polarization direction is rotated 90.degree. and thus has the same polarization of the main beam. The converted beam is thereafter passed to the light modulating LCD. However, this approach suffers from the fact that light must pass through many air-glass interfaces and reflectors which results in an inefficient system.